Food Fight: High Fat vs High Carb

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Written by Asker Jeukendrup.
Follow Asker on Twitter @Jeukendrup

There is a lot of debate about “training low”, low carb diets, Paleo diets, Atkins diets, fasted training, keto diets, etc. versus the more traditional high carbohydrate approach. It seems that people are in one of these two camps and there is little or no middle ground. The low carb group shouts, “carbohydrate is bad”, while the high carbohydrate group, you shout “you must carboload”! The purpose of this short article is to provide some clarity. We will take an evidence based approach to the questions and start to analyse the issue.

The issue gets contaminated a little by the fact that people may use certain dietary approaches for different purposes. The two extremes are: an elite athlete who wants to perform well in an endurance event and a couch potato trying to eat to lose weight or be more healthy. These are completely different purposes and it would be wrong to assume that two completely different problems should be solved with one common solution.

Let’s have a look at the theories first:

A Brief Overview of the High Carbohydrate Theory

In the 1960’s it was discovered that carbohydrate (muscle glycogen) plays an important role in fatigue. It was also discovered that carbohydrate intake during exercise can improve endurance capacity. Essentially, when carbohydrate is available, endurance is improved. When consecutive days of hard training are performed, carbohydrate will reduce symptoms of overtraining.There are many studies to support these observations and it is further supported by a clear physiological mechanism: At high exercise intensities (>80 percent of maximal oxygen uptake; VO2max), carbohydrate is the main fuel, regardless of the diet.

A Brief Overview of the High Fat Theory

Our bodies are adapted to carbohydrate because the Western diet contains a relatively large amount of carbohydrate. Therefore we have become more dependent on carbohydrate as a fuel. If we adapt to a diet that is high in fat and low in carbohydrate over the course of many weeks, our bodies adapt and will become better at oxidizing fat.There are several studies that show that a low carbohydrate diet result in increases in fat oxidation. Some of this is simply because the body is now depleted of carbohydrate, but some of this is a genuine adaptation. There are not many studies that have investigated the effects on performance. The studies performed will fit into one of these categories:

  • Short duration studies that show no changes or even decreases in performance with low carb diets

  • Long duration studies with large variability in outcomesStudies with performance measurements at very low intensities you would not find in any real life event

There is some truth in both theories although there is more evidence available supporting the carbohydrate theory, especially when it comes to performance effects. However, below are some more interesting observations:

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